1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus and more particularly to a developing device for use in an image forming apparatus.
2. Description of the Background Art
It is a common practice with an image forming apparatus to form a toner image with a photoconductive element and a developing device storing a developer. Today, a two-ingredient type developer, which is a toner and magnetic carrier mixture, is predominant over a single-ingredient type developer, i.e., toner because it makes color image formation easy. The two-ingredient type developer (simply developer hereinafter) is agitated and mixed in the developing device. As a result, toner grains are charged by friction and electrostatically deposited on carrier grains. A sleeve or developer carrier, which accommodates a magnet roller therein, magnetically attracts the carrier grains. The carrier grains deposited on the sleeve are conveyed in accordance with the rotation of the sleeve.
More specifically, the magnet roller has a main magnet for development located at a position where the sleeve is closest to the photoconductive element. When the developer approaches the main magnet, the carrier grains of the developer gather and form so-called brush chains, which constitute a magnet brush. Generally, it is considered that the carrier grains, which are a dielectric, intensify field strength between the photoconductive element and the sleeve and thereby release the toner grains from the tips of the brush chains. This is why conventional development does not use portions where brush chains are absent. More specifically, the toner grains are transferred from the brush chains to the photoconductive element only in a limited range. It has therefore been extremely difficult to increase the amount of toner grains available for development in relation to the adjustment of the other process conditions for development.
Japanese Patent No. 2,668,781, for example, proposes a developing method for implementing a high-density image in the limited range mentioned above. The developing method transfers both of toner grains deposited on brush chains of magnetic grains and toner grains deposited on a developer carrier to a photoconductive element by using an alternating electric field. However, this method limits a developing zone to a range in which magnetic grains rub against a photoconductive element. It is therefore difficult to achieve sufficiently high image density only with the toner grains deposited on the brush chains and those deposited on the developer carrier. Further, an electrode effect cannot easily implement a smooth solid image because the number of brush chains is small. Moreover, the electric field causes the toner grains deposited on the magnetic grains to move toward the sleeve and contaminate the sleeve. The contamination of the sleeve makes the electric field for development different from surrounding electric fields, causing a residual image to appear in a halftone image.
Japanese Patent Laid-Open Publication Nos. 6-208304 and 7-319174, for example, each disclose a particular developing device constructed to deposit magnetic toner grains on a photoconductive element beforehand and then remove excess toner, thereby increasing image density while reducing fog. In such a developing device, the photoconductive element accommodates a magnet. The magnetic toner grains deposited on the photoconductive element are brought into contact with an electrode roller, which also accommodates a magnet, so that needless toner grains are removed from non-image portions. A problem with this kind of developing device is that even the photoconductive element must accommodate a magnet therein. Moreover, the developing device is high cost because it is operable only with magnetic toner, and is not feasible for color image formation.
Further, Japanese Patent Laid-Open Publication No. 5-46014 teaches a developing device in which a first developing roller develops a latent image, and then a second developing roller supplied only with magnetic carrier grains removes excess toner grains. This developing device not only needs two developing rollers, but also must continuously feed only fresh carrier grains, also resulting in an increase in cost.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patents 2,668,781, 2,850,504, 3,015,116, 3,023,999, 3,077,235 and 3,084,465, Japanese Patent Publication No. 8-44214, and Japanese Patent Laid-Open Publication Nos. 60-176069, 2-173684, 5-303284, 6-324571, 8-44194, 8-44214, 8-278691, 11-338259, 2000-305360 and 2000-305361.
It is an object of the present invention to provide a developing method capable of broadening a developing zone by causing the entire sleeve to contribute to development, thereby implementing a high density, smooth solid image, and a device for practicing the same.
It is another object of the present invention to provide a developing method capable of reducing the background contamination of an image by causing the brush chains of magnetic grains to collect toner grains from the non-image portions of a photoconductive element.
A developing method of the present invention develops a latent image formed on an image carrier with a two-ingredient type developer, which consists of toner grains and magnetic carrier grains for supporting the toner grains. A developer carrier faces the image-carrier and accommodates magnetic field forming means therein for causing the developer to deposit on the developer carrier. The developer is conveyed to a developing zone formed between the image carrier and the developer carrier. The latent image is developed by a magnet brush including free toner grains, which part from brush chains formed by the carrier grains when the brush chains start rising on the developer carrier in the developing zone.
A device for practicing the above developing method is also disclosed.